Enhanced Photoluminescence and Reduced Dimensionality via Vacancy Ordering in a 10H Halide Perovskite

Vacancy-ordered halide perovskites have received great interest in optoelectronic applications. In this work, we report the novel inorganic halide Cs10MnSb6Cl30 with a distinctive 10H (10-layer hexagonal) perovskite polytype structure with (hcccc)2 stacking. Cs10MnSb6Cl30 has 30% B-site vacancies ordered at both corner- and face-sharing sites, resulting in [MnSb6Cl30]10–n columns, i.e., a reduction of octahedral connectivity to 1D. This results in enhanced photoluminescence in comparison to the previously reported 25% vacancy-ordered 3C polytype Cs4MnSb2Cl12 with 2D connectivity. This demonstrates not only the existence of the 10H perovskite structure in halides but also demonstrates the degree of B-site deficiency and stacking sequence variation as a direction to tune the optical properties of perovskite polytypes via vacancy rearrangements.


Material Characterisation
Energy Dispersive X-ray Spectroscopy (EDS) Analysis

Single Crystal X-ray Diffraction Studies (SCXRD)
X-ray diffraction data for Cs10MnSb6Cl30 were collected using a Rigaku FR-X Ultrahigh Brilliance Microfocus RA generator/confocal optics with XtaLAB P200 SCXmini CCD diffractometer [Mo Kα radiation (λ = 0.71073 Å)]. Intensity data were collected using ω steps accumulating area detector images spanning at least a hemisphere of reciprocal space. Diffraction data were collected on crystals at room temperature, 173K and 100K, and from the indexing, there was no change in structure with temperature. Following processing, the data collected at 173K was identified as the best quality, so is presented here. Data were collected using CrystalClear 1 and processed (including correction for Lorentz, polarization and absorption) using CrysAlisPro. 2 During indexing of the data, it was clear that the data were from a non-merohedrally twinned crystal. Both unit cells were identified, and data-processing took the presence of both cells into account. The twin-law relating the first cell to the second is [-1.0000 -0.0034 0.0041 -0.0006 0.5062 -1.4835 0.0001 -0.5011 -0.5059], and the refined fraction of the second cell is 0.2899(8). The structure was solved by dualspace methods (SHELXT) 3 and refined by full-matrix least-squares against F 2 (SHELXL-2018/3), 4 with anisotropic refinement of all atoms. All calculations were performed using the Olex2 interface. 5 Deposition number 2194265 contains the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures.

Powder X-ray Diffraction Studies (PXRD)
For Rietveld refinements, the background was refined using a Chebyschev function and a modified pseudo-Voigt profile function was used for peak fitting and subsequent determination of lattice parameters. For the PXRD patterns of both Cs10MnSb6Cl30 samples, data near 44.5° 2θ were excluded from refinement due to the presence of weak reflections from the sample holders.

Ultraviolet-Visible Reflectance Spectroscopy
The reflectance spectra were collected and converted to pseudo absorbance spectra by using the Kubelka-Munk transformation: where α is the pseudo absorbance and R the reflectance. 6 To estimate the band gap of Cs10MnSb6Cl30, the Tauc plot is applied based on the relationship between absorbed photon and band gap: where h is the Planck's constant, ν is the frequency of absorbed photon, Eg denotes band gap and the value of the exponent represents the nature of the electronic excitation. 7 For a direct allowed transition, the value of n is equal to 0.5. For an indirect allowed transition, the value of n is equal to 2. Tauc plots assuming both direct and indirect transitions were applied to the pseudo absorbance spectra of Cs10MnSb6Cl30 as Fig S3 illustrates.

Dielectric Measurements
Dielectric measurements were performed on a pellet of Cs10MnSb6Cl30 ca. 1.5 mm thick and 10 mm in diameter, prepared from solid state synthesised powder under uniaxial loading of ca. 1 ton. Silver electrodes were coated on the opposing pellet surfaces and dried in a drying oven at 120 °C for 1 h. Dielectric data was collected over the frequency range 100 Hz to 10 MHz with an applied AC electric field of 100mV using an Agilent 4294A in the temperature range 40 K to 475 K. Typical dielectric permittivity curves at 10 kHz and 1 MHz are shown in Fig. S4. The increase in the 10 kHz data at the highest temperatures are due to the encroaching polarisation associated with the sampleelectrode interface. The discontinuity in the data at 300 K is due to the change in sample environment for sub-and above-ambient data. No correction to the data was made to take the change in sample environment into account.

Thermogravimetric and Differential Thermal Analysis (TGA-DSC)
Thermogravimetric and differential scanning calorimetry analysis (TG-DSC) was performed using a NETZSCH STA 449 F5 system in argon (flow rate 20 ml/min) at a heating rate of 5 K/min. The temperature range was from 308 K to 673 K. TG and DSC analysis for Cs10MnSb6Cl30 solid state synthesised powder is shown in Fig. S5 (a) and (b) respectively. The initial increase at about 308K in both curves is caused by the influx of argon.